Leaf me alone

by Caroline Brown

A couple of weeks ago I had the brilliant idea of writing about why leaves change colors. Undeterred by the fact that now most of the leaves are on the ground, I’m forging forward, despite the fact that this has probably been explained on a million blogs by now.

A leaf’s green color is caused by chlorophyll, a molecule that the leaf uses to absorb energy from the sun to make food–the process called photosynthesis. Other colorful molecules exist in green leaves besides chlorophyll–carotene, an orange pigment; xanthophyll, a yellow pigment; and anthocyanin, a red pigment.

Not all trees have all pigments, though. Norway maples (Acer platanoides) have only green & yellow pigments; red maples (A. rubrum) have only greens & reds. What I would give to have one, just one, native red maple in my yard right now, instead of the sea of yellow-leaved invasive Norway maples!

During spring and summer, chlorophyll outshines all these pigments. Because spring and summer are the peak food-producing seasons, leaves constantly produce chlorophyll. As long as the tree is producing food, its leaves will be green (for the most part, realizing there are some trees with red or purple leaves or what have you).

But, as days get shorter and cooler, trees stop producing food. Chlorophyll production declines and the green color fades from the leaves as a result. The other pigments, which were there all along, now have a chance to strut their stuff.

Weather influences the intensity of fall colors. Cold temperatures slow chlorophyll production and boost anthocyanin production, as do bright sunshine and dry weather. Therefore, the most intense fall colors can be expected in dry periods where sunny days are followed by cool nights.

So, voila….better late than never: that’s why leaves turn from green to orange, yellow & red in the fall.

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7 Responses to “Leaf me alone”

Well I haven’t read the explanation elsewhere (at least not that I can remember, probably a long time ago), thanks for explaining this process and of course we can all be thankful for the amazingly colorful results.

“What I would give to have one, just one, native red maple in my yard right now, instead of the sea of yellow-leaved invasive Norway maples!”

We have a huge Norway maple behind the garage (it’s leaning on the garage and deforming the roof, so clearly it was one of those trees that snuck in and got large before anybody woke up to what was happening). I’ve cleaned leaves out of the garden beds twice now and heading out for the third round today. No more leaves on the tree, so the only thing left is scatter from the neighbors’ yards.

Fortunately, along the street there are some sugar maples that turn brilliant orange, so there is something cheery to look at while raking.

Last year I read an interesting science news item from Colgate University about the anthocyanins in leaves:

“When scarlet-tinted autumn leaves are dropped in the fall, it appears that anthocyanins leach from the leaves into the soil and protect seedlings and saplings from interspecific competition the following spring,” said Frey. “This seems a viable possibility, since the molecular structure of anthocyanin is nearly identical to catechin, a well-described toxin that causes root cells to self-destruct.”

that’s an interesting bit about anthocyanins. I’ve heard all sorts of evolutionary tricks that have been developed by both natives and invasives for outcompeting other species but was not aware of this one. Fascinating!